The influence of productivity and diversity on the invasibility of a phytoplankton community

Biological invasions are one of the most important threats of biodiversity, hence understanding the underlying mechanisms of invasibility (i.e. susceptibility of an environment to invasion by new species) is crucial. Invasibility can be affected by various factors such as 1) productivity, 2) diversity, and 3) the characteristics of the invasive and resident species. Using semi-continuous microcosms, we investigated the effect of productivity and diversity on the invasibility of a phytoplankton community in the presence or absence of an herbivore by the introduction of two functionally different algal species. The productivity gradient was generated by manipulating the nutrient concentration in a particular community assembly. With increasing productivity herbivory increased and invasibility by both invaders showed a unimodal pattern. At low productivity the invasibility mainly depended on the competitive abilities of the invaders, whereas at high productivity the susceptibility to herbivory dominated. Due to their species specific characteristics, the maximum of the unimodal pattern along the productivity gradient differed between the two invading species. The diversity gradient was generated by manipulating the number of randomly selected species at an intermediate productivity level. Invasibility was not affected by diversity; instead it was driven by the functional characteristics of the resident and/or invasive species mediated by herbivore abundance. In conclusion, our results suggest that productivity has a major impact on the invasibility in phytoplankton communities with respect to the characteristics of the invaders, while diversity plays a minor role.